Four techniques are known for creating demultiplexing in integrated optics: the first technique uses an etched grating, the second uses Mach-Zehnder interferometers, the third uses a phase or PHASAR network (i.e. PHASe-Aray) and the fourth uses balanced Mach-Zehnder interferometers or 100% couplers with identically photo-etched Bragg gratings on the two arms ("ADD-DROP multiplexer").
The first technique uses light diffraction with a concave grating with a circular or flat output field that is etched and blazed to a high level.
Vertical etching is possible when silica guides are on silicon and may reach a depth of 25 .mu.m.
Document (6) may be referred to for further information about this subject.
The demultiplexer component then consists of an input fiber connected to a planar guide that sends the light in the direction of an etched diffraction grating.
In the example of a circular output field grating, the incident ray and the diffracted light that are refocused at various angles of incidence, are localized on the Rowland circle.
In the example of a flat field grating (see document (6)), the stigmatic points dispersed in wavelengths are aligned at an orthogonal right angle to the reflected beam.
Given that the grating operates using reflection, it is metal-coated.
The shape of the grating etching can be constituted by a number of ellipses, as shown in document (7).
The diffracted beam is refocused on single-mode guides, for example with a mode diameter of 9 .mu.m and a spacing of 16 .mu.m as shown in document (6), or on photodiodes that create a strip, as shown in document (5).
The grating preferably operates at a high order of diffraction, ranging from 4 in document (6) to 50 in document (5), to achieve high density demultiplexing for telecommunications.
The second technique is based on using several Mach-Zehnder or similar interferometers in series. These interferometers are all unbalanced in terms of their optical paths with a characteristic imbalanced value.
Document (8) may be consulted for further information about this subject.
For a four-channel demultiplexer two interferometers are used, for example, the imbalances of which are respectively .DELTA.L.sub.1 and .DELTA.L.sub.2 =.DELTA.L.sub.1 +.lambda./4N, and a third interferometer the imbalance .DELTA.L.sub.3 of which is equal to 2. .DELTA.L.sub.1 (usually of the order of between 50 .mu.m and 100 .mu.m) in order to obtain a separation between the channels of between 7.5 nm to 1,550 nm, N being the effective index of the mode.
The third technique uses an optical phase-array grating that comprises a number of parallel single-mode phase shifter guides that connect two flat input and output guides with circular interfaces.
Document (9) may be consulted for further information about this subject.
The input and output guides are connected to the other circular interfaces of the flat guides.
The light injected by any of the input guides is dispersed in the flat input guide and covers all the phase shifter guides located at the interface.
There is a constant difference in length between one phase shifter guide and another such that the beams of light leaving the outlet of the flat guide interfere as though they were reflected by a concave sloped diffraction grating.
The shift in the optical path induced by the phase shifter guides produces the same effect as a slope in the leading edge of the wave relative to the interface.
The PHASAR, which operates by transmission, therefore behaves like a concave diffraction grating of a very high order (approximately 50 to 100) and of a high multiplexing capacity.
Document (10) may be consulted for further information about this subject.
Better spectrum definition is achieved using a greater number of phase shifter guides.
In document (11), for example, 60 phase shifter guides are used.
A half-wave plate can be inserted in the center of the optical circuit constituted by the phase shifter guides in order to eliminate the dependence of the circuit on polarization.
The fourth technique uses balanced Mach-Zehnder interferometers or 100% couplers with Bragg gratings identically photo-etched on the two arms. For all the distinct wavelengths of the Bragg wavelength the light is injected at port 1 and is emitted at port 3 (100% coupling); the Bragg wavelength light is selectively reflected at port 2. Document (29), from where the number references above are taken, may be consulted for further information about this subject.
Three types of material are used to produce the components that are used in the four techniques above: glass, silica on silicon and InP or similar semi-conductors.
In particular, etched gratings and PHASARs have been produced using integrated optics on silicon whereas demultiplexers with interferometers have been made using integrated optics on silicon or glass.
None of these known techniques enable the Bragg wavelengths to be directly determined with satisfactory accuracy.
Also, these techniques require a compromise to be made between cross talk and occupied spectrum space.
Cross talk, i.e. the coupling of light between the outputs, should be minimized because it leads to the wavelengths being inaccurately measured.
Typically, -25 dB to -30 dB cross talk is preferred and the spectrum occupation is consequently deduced.
When a diffraction grating uses integrated optics on silicon the light coupling between the outputs is induced by the diffusion in the guide, due to the etching imperfections, and by the coupling between the output guides when they are too close together.
The cross talk is generally of the order of between -20 dB and -35 dB between the centers of two adjacent spectrum channels whereas it only ranges between -10 dB and -15 dB at the intersection of the transfer operations of these channels, at the mid-point of the spectrum period.
PHASARs produce some of the best cross talk and occupied spectrum space characteristics.
Generally, cross talk better than -30 dB is achieved in document (11) where the spectrum occupation is 0.8 nm and the period is 2 nm, using 60 phase shifter guides, and where the order of diffraction is 60.
In Mach-Zehnder interferometers, the cross talk is dependent on the accuracy of the setting of the 3 dB separation couplers.
As an example, document (8) describes a demultiplexer that is constituted by three interferometers comprising 3.1 dB couplers, instead of 3 dB couplers, and that is characterized by approximately -20 dB cross talk.
Document (4) also describes a demultiplexer that includes a collimation device for the light to be analyzed and a series of bandpass filters that are assembled in cascade and associated with photodetectors.
The main drawback with this demultiplexer is that it is designed to operate in an open space.
This results in the reproducibility and the reliability of the measurements, as well as the robustness and the integration of this demultiplexer, being insufficient for use in micro-system applications.
Furthermore, the minimal cross talk that it is possible to obtain with this demultiplexer is dependent on the reflection of the bandpass filters used, that typically comprise -20 dB anti-reflection deposits, and is also highly dependent on the polarization of the light analyzed (the filters are at a 45.degree. angle).
Finally, this type of demultiplexer is not suitable to be industrially produced to meet the requirements of the industrial sensor market.